424 



MECHANICAL AND ACOUSTICAL SENSES 



O <J Q y 



f) twilight 



L 



night 



i 



2000 



meters 



start 





100 meters^. 



£* >-: *■ , ' .T T , 



Isthmus Cove 



-i^" SANTA '-■. 

 CATALINA ISLAND 



Figure 2b Individual track (approx. 0.5-h intervals) of one of the above sharks, starting 

 at 1200 h, ending at 0600 h. (From Sciarrotta and Nelson 1977.) 



movements, or pressure fluctuations in the gastrointestinal tract. Techniques 

 of this type are treated by Mackay (1970) and by Kanwisher et al. (1974). 

 An example of continuous-transmission FM telemetry from a shark is the 

 study of Kotchabhakdi et al. (1973), who monitored electrical activity from 

 the brains of free-swimming dogfish in a large pen. 



Because this article addresses mainly the problems of long-range telemetry 

 from free-ranging sharks in the natural environment, only techniques of 

 pulsed transmission will be discussed further. 



Types of Ultrasonic Transmitters 



Figure 3 diagrams the signal parameters used in the following discussion of 

 pulsed ultrasonic transmitters (USTs). Generally, when sensor data have 

 been transmitted, they have been encoded as variations in pulse rate (pulse 

 interval). Pulse-length variation is a poor way to encode data, as propagation 

 of the sound through the water can time-stretch and distort the received 

 pulse, especially if multipath problems are serious. Frequency variation has 

 likewise not been used as a means of encoding sensor data from small, pulsed 

 USTs, although it is used to distinguish between individual transmitters (each 

 at a different frequency) or between channels in some two-channel trans- 

 mitters. 



